Mass spectrometry is an analytical technique used for identifying the composition of a substance based on the mass of the substance or of constituent atoms or molecular fragments into which it may be broken. It is particularly valuable because it frequently requires little information about the sample submitted for analysis to obtain useful results. For example in the qualitative analysis of a sample under test containing metal atoms, the constituent metal atoms and isotopes can be identified by their atomic mass.
In a mass spectrometry experiment it is normally necessary to make the sample under test available to the inductively coupled plasma ionization system as a vapor or aerosol then ionize the material so that its mass may be analyzed by accelerating the ions using an electric or combination of electric and magnetic fields. The mass of the constituent parts is determined from the ratio of the mass of the particle to its charge. The substance or chemical constituent that is of interest in an analytical procedure is referred to as an analyte. The material to be analyzed in the form obtained initially by the lab is referred to as the “sample submitted for analysis”. The material to be analyzed in a form ready for analysis is referred to as the sample under test. Frequently the results of the analytical procedures on a first sample under test are compared to the results of those same procedures on a second sample of known concentration. This second sample of known concentration is referred to as a standard. Standards may be in the form of separate samples under test or they may be added to the sample as submitted before analysis in which case they are known as internal standards.
Inductively coupled plasma (ICP) is a technique for ionizing a vaporized sample. It is used in the art for elemental determinations by mass spectrometry. ICP has been used as an ionization technique with a variety of analytical techniques such as atomic emission spectroscopy. When the ionization method of ICP is used as the ionization method for a subsequent analysis by mass spectrometry the technique is referred to as inductively coupled plasma mass spectrometry (ICP-MS). ICP-MS typically requires introduction of a sample into the plasma discharge as an aerosol. The plasma discharge converts the aerosol into elemental ions which can be analyzed by the mass spectrometer. Sometimes an additive is introduced after the aerosol is formed to insure the aerosol is completely ionized by the inductively coupled plasma. In the exemplary embodiment shown this is a dilute solution of a highly oxidizing acid such as 2% nitric acid.
Sometimes a technique known as isotope dilution is used in connection with ICP-MS. Isotope dilution ICP-MS is based on the addition of a known amount of an enriched isotope (called the “spike”) to a sample. After equilibration of the spike isotopes with the natural isotopes in the sample, mass spectrometry is used to measure the altered isotopic ratio(s). The concentration is directly derived from this ratio.
Laser ablation (LA) is a technique known in the art used to make an aerosol that can be transported by an inert carrier gas such as helium into the plasma discharge. Laser ablation has been used with liquid and solid samples and compliments the ICP ionization technique. Normally a laser is focused on a sample under test within an ablation chamber. The light from the laser heats the sample until it ablates, and the resulting aerosol is swept into the inductively coupled plasma (ICP) chamber by an inert carrier gas such as helium. The results of the laser ablation process are highly dependent on the sample however. Different samples under test absorb different amounts of the laser light, and so for some samples only a surface layer is removed, and for other samples the ablation penetrates more deeply into the sample.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is considered one of the most powerful techniques for identifying trace elements in sold materials. LA-ICP-MS uses a laser to ablate or vaporize a portion of the sample on or near the surface the laser is focused on. LA-ICP-MS is used for analysis of solid samples in geological, archeological, environmental and biological studies. LA-ICP-MS is fast, sensitive and able to probe micro-scale features.
However, there are limitations known in the art with respect to LS-ICP-MS in that detection capabilities will vary with the sample matrix, which may affect the degree of ionization that will occur in the plasma or allow the formation of species that may interfere with the analyte determination.
Another problem known in the art with LA-ICP-MS is the susceptibility of the process to errors caused by contamination, matrix effects and inter-element fractionation. Each of these phenomena compromise attempts at standardization and makes accurate quantification hard to achieve. Many researchers have developed complicated strategies to overcome these obstacles such as the use custom matrix matched standards, liquid ablation, dual introduction (sample+standard), and isotope dilution. All these methods are time consuming, and many are difficult and costly as well. Custom matrix matched standards involves making up several samples continuing standards in the matrix of the sample. In this technique a different set of standards or a different matrix needs to be used for each sample or sample type analyzed. This process of preparing a different standard for every analysis is cumbersome, tedious, and expensive. Liquid ablation has the same issues as other custom standard method and additionally requires special sample handling to minimize splashing and other effects of the ablation process. Isotope dilution as taught in the art requires the addition of isotope enriched standards for every element suspected of being present in the sample.
The method of standard addition is used in instrumental analysis to determine concentration of a substance in an unknown sample by comparison to a set of samples of known concentration, similar to using a calibration curve. However, because the results of LA-ICP-MS are known to be sensitive to matrix effects, contamination, and inter-element fractionation, it is appreciated in the art that the method requires a time consuming process of preparing mixtures of sample and standards. The method of standard addition can be implemented by preparing several samples for analysis containing the same amount of unknown, but different amounts of standard. The idea of this procedure is that the total concentration of the analyte is the combination of the unknown and the standard, and that the total concentration in the set varies linearly. If the signal response is linear in this concentration range, then the relative amounts of the constituents in the sample are solved for using equations containing the known amounts of the standard and the relative amounts of the constituents in the standard.
However, in the case of most LA-ICP techniques including LA-ICP-MS the sample preparation usually includes a tedious process of spiking, homogenizing, drying and pelletizing steps that need to be repeated for each additions of a single sample. Nevertheless, this method has been successfully used for the analysis of viscous crude oil, zircons fused in glass and hardly digestible samples prepared with a gluing technique.
An online solution based addition developed for laser ablation microprobe ICP-MS and then for LA-ICP-MS analysis and been shown to minimize the time consuming nature of standard additions when used with laser ablation. This method consists of performing the addition by mixing the ablated sample with a nebulized elemental solution using a Y connection. A similar approach involves mixing the aerosol coming from an ultrasonic nebulizer with the ablated particles directly inside the ablation cell. Even though these techniques have shown promising results they require researchers to identify at least one element in advance of the analysis and prepare an appropriate standard solution for that element, and/or factor in the ablation rate for each element. The ablation rate may or may not be known with the appropriate accuracy to make this technique possible.
It is desirable to have the ability to add a test component to a sample to calibrate the testing method, account for impurities, and eliminate matrix effects. However, doing so in conjunction with laser ablation involves a time-consuming sample preparation process for solid sample types. Various sample preparation methods known in the art as spiking, homogenizing, drying and pelletizing steps need to be repeated for each additions of a single sample. These methods have been successfully used for the analysis of viscous crude oil; zircons fused in glass and hardly digestible samples prepared with a gluing technique, but are cumbersome, tedious, and expensive.
Recently a new approach implying the mixing of two aerosols has been investigated. This technique is based on a system equipped with a galvanometric scanning beam device that allowed the laser to rapidly move with high repositioning precision. Using such a device, the quasi-simultaneous ablation of the sample and an isotopically enriched pellet was realized, thus performing the isotope dilution (ID) directly inside the ablation cell. However, this method requires the use of specialized instrumentation, expensive isotopically-enriched standards, and assumes that the same amount of material has been ablated between the sample and the spike pellet.
It is desirable to have a convenient, inexpensive method for successfully introducing an additive of known composition in the same matrix as the analyte which may be used to implement the methods of standard addition or isotope dilution for LA-ICP-MS. It is further desirable to have material which can serve as standards be reusable over many analyses.